The present invention relates to a polysilicon evaluating method of evaluating states of crystals of a polysilicon film and a polysilicon film evaluating system used therefore, and a thin film transistor fabricating method of fabricating a thin film transistor having a polysilicon film formed by annealing amorphous silicon and a thin film transistor fabricating system used therefore.
In recent years, thin film transistors using polysilicon films as channel layers have come to be put into practical use. A thin film transistor using a polysilicon film as a channel layer exhibits a very high field mobility, and accordingly, if being used as a drive circuit for a liquid crystal display or the like, such a thin film transistor can realize higher definition, higher operational speed, and miniaturization of the display.
On the other hand, in recent years, a so-called low temperature polycrystallization process has been developed. In this process, a polysilicon film is formed by heat-treating amorphous silicon by using an excimer laser annealing system. In the case of applying such a low temperature polycrystallization process to fabrication of thin film transistors, since a thermal damage to a substrate becomes low, it is possible to use an inexpensive glass substrate having a large area.
However, since an output power of an excimer laser annealing system used for the low temperature polycrystallization process is unstable, grain sizes of polysilicon formed by laser annealing largely vary depending on the unstable output power. As a result, grain sizes of crystals of a polysilicon film formed by using the excimer laser annealing system is not necessarily desirable. For example, if crystals of the polysilicon film thus formed have micro grain sizes, there arises a problem associated with a so-called linear failure, and if crystals of the polysilicon film have grain sizes not sufficiently large, there arises a problem associated with a so-called writing failure.
Accordingly, in the case of forming polysilicon films for a number of devices by annealing using such an excimer laser annealing system, after the end of the polycrystallization step for the polysilicon films, the devices on which the polysilicon films have been formed are generally subjected to total inspection or random sample inspection in terms of states of crystals of the polysilicon film formed on the outermost surface of each of all the devices or the randomly sampled devices, and at that stage, it is decided whether or not the devices thus semi-finished are defective, and information on an energy of a laser beam given from the excimer laser annealing system to amorphous silicon from which the polysilicon film is formed is fed back to the excimer laser annealing system, to set a laser power to an optimum value.
However, as a method of evaluating a polysilicon film, there is known only a sensible method of picking up a surface image by using a spectral ellipsometer, a scanning electron microscope or the like and judging states of crystals of the polysilicon film by visually observing the surface image thereof. Such a method fails to objectively judge states of crystals of a polysilicon film in a non-contact manner, and is not efficient in terms of time and cost. As a result, the method is difficult to be used for evaluation of the polysilicon film in-process.
An object of the present invention is to evaluate a state of a polysilicon film objectively, accurately, automatically, and in a non-contact manner.
To achieve the above object, according to a first aspect of the present invention, there is provided a polysilicon evaluating method of evaluating a polysilicon film formed by annealing an amorphous silicon film, including the steps of: picking up an image of a surface of the polysilicon film; dividing the picked-up image into a plurality of regions and calculating a contrast in each of the regions divided from the picked-up image; detecting a high contrast region and a low contrast region and comparing the contrasts in the high contrast and low contrast regions with each other; and evaluating a state of the polysilicon film on the basis of the comparison result.
According to a second aspect of the present invention, there is provided a polysilicon evaluating system for evaluating a polysilicon film formed by annealing an amorphous silicon film, including: pick-up means of picking up a surface of the polysilicon film; and evaluation means of dividing the picked-up image into a plurality of regions, calculating a contrast in each of the regions divided from the picked-up image, detecting a high contrast region and a low contrast region, comparing the contrasts in the high contrast and low contrast regions with each other, and evaluating the state of the polysilicon film on the basis of the comparison result.
According to a third aspect of the present invention, there is provided a thin film transistor fabricating method of fabricating a thin film transistor, including: amorphous silicon forming step of forming an amorphous silicon film; polysilicon film forming step of forming a polysilicon film by annealing the amorphous silicon film; and evaluating step of picking up an image of a surface of the polysilicon film, dividing the picked-up image into a plurality of regions, calculating a contrast in each of the regions divided from the picked-up image, detecting a high contrast region and a low contrast region, comparing the contrasts in the high contrast and low contrast regions, and evaluating the state of the polysilicon film on the basis of the comparison result.
According to a fourth aspect of the present invention, there is provided a thin film transistor fabricating system for fabricating a thin film transistor, including: an amorphous silicon forming device for forming an amorphous silicon film; a polysilicon film forming device for forming a polysilicon film by annealing the amorphous silicon film; and an evaluating device for picking up an image of a surface of the polysilicon film, dividing the picked-up image into a plurality of regions, calculating a contrast in each of the regions divided from the picked-up image, detecting a high contrast region and a low contrast region, comparing the contrasts in the high contrast and low contrast regions, and evaluating the state of the polysilicon film on the basis of the comparison result.
According to the polysilicon evaluating method and the polysilicon evaluating system of the present invention, it is possible to evaluate a state of a polysilicon film objectively, accurately, automatically, and in a non-contact manner.
According to the thin film transistor fabricating method and the thin film transistor fabricating system, it is possible to easily inspect a polysilicon film in a nondestructive manner, and hence to incorporate the inspection step in the fabrication process. Also, since the inspection can be performed on the basis of numerical calculation without the need of visual inspection, it is possible to automate the inspection and to objectively perform the inspection at a high accuracy. Further, it is possible to enhance a fabrication yield of thin film transistors by feeding back am inspection result to an annealing step.